When it comes to covering up a flat roof, your options are both limited and expansive. What that means in a nutshell is that your traditional roofing materials such as asphalt shingles, concrete tiles, and corrugated metal are out of the window. That being said, flat roof systems such as PVC, TPO, EPDM rubber, and others, each offer their pros and cons.
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So why can’t you put traditional roofing materials on a flat roof? Well, technically you can, but they are almost guaranteed to leak. Systems like asphalt shingles and concrete tiles are installed by overlapping one row on top of another. They work cohesively with the pitch of the roof to shed rainwater and snow as it falls from the sky. Since flat roofs have little to no pitch, the water would work itself underneath the shingles or tiles, eventually rotting the substrate and causing leaks on your interior.
Covering a flat roof is a whole different animal than shingling a pitched one. On a flat roof, generally speaking, you want to avoid any types of seams if at all possible. The biggest threat is of course going to be water, which WILL find any access though any hole or inadequately-sealed seams in the roofing membrane. Your main goal when covering a flat roof is to create a barrier that will be impenetrable to water.
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How do you make an item (besides a roof) impenetrable to water? You can either apply something physical such as a tarp, or coat it with a material to create a barrier like you would via deck stain or lacquer. Roofs follow this same premise – either physically cover it with something like a PVC membrane or apply a coating such as tar or spray on silicon.
Why Do Flat Roofs Exist?
Before we get into comparisons of different flat roofing materials, it’s important to know why they exist in the first place since they’re seemingly such a hassle and an almost-immanent leak threat. There are two main reasons behind why a contractor would call for a flat roof – 1) aesthetics and 2) convenience. For example, when you’re adding on to a home (building an addition) with something like a three-seasons room, a flat roof simply looks nicer. Homes with unblended roof pitches can be an awkward eyesore. In commercial buildings, flat roofs, outright, offer a more convenient place to install outdoor HVAC units rather than putting them in high-traffic ground areas.
Of course, any roof’s main job is to create a barrier of protection between the building below and the atmosphere above. Therein lies the conundrum associated with flat roofing. For all the aesthetics and convenience, the design doesn’t do a lot to avoid snow and water buildup.
To be fair, flat roofs aren’t completely ‘bubble-level’ flat. They work in much the same way as a gutter system, angled slightly or pitched a couple of degrees, so that water flows into a downspout. Even so, flat roofing materials need to be able to absorb the brunt of the weather and to withstand ponding water or snow and ice until it melts.
Pros and Cons of Common Flat Roof Membranes
Whether you have a commercial or residential building, some type of roof covering is 100% mandatory. There are 5-6 different routes to take regarding the materials needed for roofing a flat surface:
- PVC Single-Ply Membrane – arguably the most popular of all flat roofing materials (some contractors even install ONLY PVC membranes.) PVC membranes are a single layer of thermoplastic material. PVC roofs are especially strong and durable, featuring a minimum breaking point of 300 pounds per inch compared to the industry-recommended 200 PPI. Seams are heat-welded (hot air welded) to form a watertight bond. The welded seams are actually stronger than the material itself! Most PVC membranes, especially white-colored ones are very energy efficient, because they reflect the sun’s energy instead of absorbing it. The PVC membranes often come with lifetime warranties.
- EPDM – commonly known in the industry as a rubber roof. One of the biggest advantages of EPDM over a PVC roof membrane is a much lower price. PVC and EPDM typically go 1-2 in terms of popularity for flat roofing depending on what area you live in. The seams of EPDM are not as strong as the PVC, heat-welded joints either. EPDM also has a tendency to absorb heat, which can drive up utility bills for the property owner.
- TPO – another single-ply roofing membrane that chemically bonds rubber, ethylene, and propylene as well as numerous filler materials. TPO roofs have been vastly growing in popularity ever since their introduction in the early 1990s. Part of what is fueling their recent rise is energy-efficiency, as they are more readily available in white and light gray (as well as black) compared to their closest competitors PVC and EPDM. TPO is said to offer the best of both EPDM and PVC in one package. The material is closer to a rubber roof in cost, but also features welded seems for durability like PVC membranes.
- Modified Bitumen – one negative of TPO, EPDM, and PVC membranes is, although they are durable, sharp objects and collisions can still rip through the single ply of the membrane. Modified bitumen features an advantage in this aspect since it is a multi-ply roofing material. A base layer is mechanically attached to the roof deck with plates or bars. A ply overlap is then sealed to the base layer with a permanent adhesive. Finally a granule top surface is applied to provide aesthetic and energy efficient properties.
Credits: Commercial Roof USA